Sense Organs

Sense Organs
Animals require a constant inflow of
information from the environment to
regulate their lives. Sense organs are
specialized receptors designed for
detecting environmental status and
change. An animal’s sense organs are
its first level of environmental perception;
they are channels for bringing
information to the brain.

A stimulus is some form of energy
—electrical, mechanical, chemical, or radiant. A sense organ transforms
energy from a stimulus into nerve
impulses, the common language of
the nervous system. In a very real
sense, then, sense organs are biological
transducers. A microphone, for example,
is a transducer that converts
mechanical (sound) energy into electrical
energy. Like the microphone,
which is sensitive only to sound, sense
organs are, as a rule, specific for one
kind of stimulus. Thus eyes respond
only to light, ears to sound, pressure
receptors to pressure, and chemoreceptors
to chemical molecules. But
again, all forms of energy are converted
into nerve impulses.

Since all nerve impulses are qualitatively
alike, how do animals perceive
and distinguish different sensations
of varying stimuli? The answer is
that real perception of sensation is
done in localized regions of the brain,
where each sensory organ has its own
hookup. This concept of “labeled
lines” of communication to specific
brain regions was first described by
Johannes Müller in the 1830s, who
called this the law of specific nerve
energies. Impulses arriving at a particular
sensory area of the brain can
be interpreted in only one way. For
example, pressure on the eye causes
us to see “stars” or other visual patterns;
mechanical distortion of the eye
initiates impulses in the optic nerve fibers that are perceived as light sensations.
Although such an operation
probably could never be done, a
deliberate surgical switching of optic
and auditory nerves would cause the
recipient literally to see thunder and
hear lightning!